The present invention pertains to improvements in the field of wastewater treatment. More particularly, the invention relates to a biofiltering device for treating wastewater discharged from a septic tank.
Small wastewater treatment systems are typically designed so that raw wastewater generated by a residence is discharged directly into a septic tank. Once discharged into the septic tank, the effluent is allowed to partially settle and is then passed into a dosing chamber. From the dosing chamber, the effluent is fed into a filtering medium, e.g. a trickling bed filter, and then collected in a central drain pipe. Once the effluent is collected in the central drain pipe, it is then dispersed to a plurality of parallel drain pipes positioned generally equidistant from one another across a specified area underneath the ground. Each drain pipe has a plurality of orifices which allow the effluent to be released into the surrounding environment.
Various filtering media and devices have been proposed for treating the wastewater discharged from a septic tank. For example, U.S. Pat. No. 5,206,206 discloses the use of pre-treated peat in biofilters for wastewater treatment. The pre-treated peat comprises a mixture of pre-sieved peat with a Fe-containing compound and lime. A filter-bed constituted of a layer of such a pre-treated peat disposed between upper and lower layers of calcareous stones is arranged inside a filtration column. A rotary distribution system including a perforated arm is used for distributing the wastewater to be treated on top of the filter-bed. Since the pressure of the incoming wastewater serves to displace the perforated arm and the wastewater entering the distribution system contains suspended particles of organic and inorganic materials, the suspended particles often clog the rotation mechanism, thus rendering the distribution system inoperative.
U.S. Pat. No. 5,618,414 also discloses a wastewater treatment system utilizing peat as a filtering medium. Such a treatment system comprises a container having an upper portion, a lower portion, at least one waster water inlet in the upper portion of the container for receiving the wastewater, and an opening in the lower portion of the container for allowing the treated water to escape the container. At least one elongated hollow casing is mounted within the lower portion of the container. The casing has an open bottom surface and defines at least two treatment chambers within the container, each of the treatment chambers containing a bed of peat for treating the wastewater. At least one distribution means is also provided for distributing the wastewater entering the container through the wastewater inlet into at least one of the treatment chambers. Each distribution means comprises a water inlet pipe connected to the wastewater inlet and leading above the casing and a trough tiltably mounted on top of a corresponding casing. The trough extends along the casing and has two opposite sides. It defines at least one wastewater receiving means on one of the two sides and it is tiltable between a first position whereat the wastewater receiving means receives wastewater exiting the water inlet pipe and a second position whereat the wastewater received in the receiving means flows out of the same. The trough also has counterweight means on its other side for holding it in the first position while it is filled up and for bringing it back from the second position to the first position after the at least one wastewater receiving means has been emptied. In addition, at least one distribution plate is mounted above the bed of peat in one of the treatment chambers defined by the corresponding hollow casing. This at least one distribution plate comprises a plurality of channels projecting from the trough, each of the channels having an end for receiving wastewater flowing from the trough so that the wastewater is divided into a plurality of flows each flowing in a corresponding channel, each of the channels also having at least one opening for letting the wastewater drip into the corresponding treatment chamber.
The use of the aforesaid tiltable trough in combination with the distribution plate for distributing the wastewater over the bed of peat presents certain drawbacks. When the wastewater enters the container at a high flow rate and a large amount of wastewater is continuously received in the trough over a period of time, the trough rapidly tilts between the aforesaid first and second positions so that the wastewater flowing therefrom floods the channels defined in the distribution plate and flows over the edges of the distribution plate and onto the bed of peat. There is thus flooding of the bed of peat. Since any given peat has a predetermined biofiltering capacity over which the peat will not act as a biofiltering medium, flooding of the bed of peat will cause the wastewater to flow through the bed of peat at a flow rate greater than the biofiltering capacity of the peat, resulting in a non-efficient removal of both organic and inorganic materials.
It is therefore an object of the present invention to overcome the above drawbacks and to provide an improved biofiltering device which utilizes peat as biofiltering medium and which can efficiently treat wastewater irrespective of the flow rate thereof.
In accordance with the present invention, there is provided a biofiltering device for treating wastewater, comprising a housing having inlet means for receiving the wastewater to be treated and outlet means for discharging the treated wastewater, a bed of peat disposed inside the housing between the inlet and outlet means, the peat defining a biofiltering medium having a predetermined biofiltering capacity, and means for aerating the peat. The device according to the invention further includes a fluid flow control system arranged over the bed of peat and in fluid flow communication with the inlet means, for distributing the wastewater through the bed of peat in a manner such that when the wastewater enters the system at a flow rate greater than the biofitering capacity of the peat, the wastewater discharged from the system flows through the bed of peat at a flow rate no greater than the biofilteting capacity.
According, to a preferred embodiment of the invention, the fluid flow control system comprises a fluid flow control unit having a chamber of variable volume in fluid flow communication with the inlet means and a plurality of spaced-apart discharge orifices in fluid flow communication with the chamber, the volume of the chamber varying as a function of a difference between the flow rate of the wastewater entering the chamber and the flow rate of the wastewater discharged therefrom and increasing when the flow rate of the wastewater entering the chamber is greater than the biofiltering capacity of the peat. The discharge orifices each have a dimension selected so that the wastewater discharged from the chamber flows trough the bed of peat at a flow rate substantially equal to the biofiltering capacity of the peat when the flow rate of the wastewater entering the chamber is equal to or greater than the biofiltering capacity.
In a particularly preferred embodiment, the fluid flow control unit comprises an elongated, horizontally extending receptacle having upper and lower walls formed of a flexible material, the lower wall being provided with the aforesaid orifices and lying on the bed of peat. The chamber is defined between the upper and lower walls with the upper wall being movable towards or away from the lower wall in response to a decrease or increase in the volume of the chamber. The receptacle further has a feed inlet in-fluid flow communication with the inlet means for feeding the wastewater into the chamber. Preferably, the feed inlet is disposed at one end of the receptacle and wherein the lower and upper walls each have a predetermined width and the orifices are formed in the lower wall at predetermined locations, the width and locations being selected so as to cause the wastewater discharged through each orifice to flow at a substantially uniform flow rate. The orifices are generally circular and each have a diameter ranging preferably from about 2 to about 8 mm. For example, when the peat used is sphagnum peat having a biofiltering capacity of about 20 l/hr, the orifices each have a diameter of about 6 mm. When using sphagnum peat having a biofiltering capacity of about 22 l/hr, the orifices each have a diameter of about 5 mm. In the case of sphagnum peat having a biofiltering capacity of about 24 l/hr, the orifices each have a diameter of about 3 mm.
According to another preferred embodiment, the fluid flow control system comprises first and second fluid flow control units with the first unit being disposed on top of the second unit, the first fluid flow control unit being adapted to control the flow rate of the wastewater discharged therefrom and the second fluid flow control unit adapted to receive the wastewater discharged from the first unit and cause the wastewater to flow throughout substantially the entire bed of peat.
Preferably, the first fluid flow control unit has a chamber of variable volume in fluid flow communication with the inlet means and a plurality of spaced-apart discharge orifices in fluid flow communication with the chamber, the volume of the chamber varying as a function of a difference between the flow rate of the wastewater entering the chamber and the flow rate of the wastewater discharged therefrom and increasing when the flow rate of the wastewater entering the chamber is greater than the biofiltering capacity of the peat. The discharge orifices each have a dimension selected so that the wastewater discharged from the chamber flows though the bed of peat at a flow rate substantially equal to the biofiltering capacity of the peat when the flow rate of the wastewater entering the chamber is equal to or greater than the biofiltering capacity.
In a particularly preferred embodiment, the first fluid flow control unit comprises an elongated horizontally extending receptacle having upper and lower walls formed of a flexible material, the lower wall being provided with the aforesaid orifices and lying on the second fluid control unit. The chamber is defined between the upper and lower walls with the upper wall being movable towards or away from the lower wall in response to a decrease or increase in the volume of the chamber. The receptacle farther has a feed inlet in fluid flow communication with the inlet means for feeding the wastewater into the chamber.
According to a further preferred embodiment, the second fluid flow control unit comprises an elongated, horizontally extending porous membrane having upper and lower surfaces with the lower surface contacting the peat, the lower wall of the receptacle lying on the upper surface of the membrane. The membrane is capable of spreading the flow of the wastewater discharged from the receptacle as the wastewater flows through the membrane from the upper surface to the lower surface, and into the bed of peat. Preferably, the membrane is a multilayered membrane comprising upper and lower layers formed of non-woven fibers and each having a predetermined density, and an intermediate layer disposed between the upper and lower layers and formed of the non-woven fibers, the intermediate layer having a density less than the predetermined density. The upper and lower layers each have a plurality of spaced-apart apertures extending therethrough and formed, for example, by piercing the upper and lower layers with needles.
According to still a further preferred embodiment, the housing has a bottom opening defining the outlet means and a metal grating covers the bottom opening to support the bed of peat inside the housing while enabling the treated wastewater to flow through the bottom opening.
According to yet another preferred embodiment, the biofiltering device further includes sampling means enabling a sample of the treated wastewater to be collected for analysis. Preferably, the sampling means comprise a horizontal tray-like member disposed inside the housing adjacent a sidewall thereof and the bottom opening, and a guide member connected to the tray-like member and extending through an aperture formed in the sidewall. The tray-like member has a main fluid-receiving surface extending along an inclined plane for causing drops of the treated wastewater received on the main surface to flow in a direction towards the sidewall and the guide member has a guide channel arranged to receive the drops of treated wastewater from the main surface for guiding the drops through the apertures and exteriorly of said housing. The tray-like member preferably has two secondary fluid-receiving surfaces disposed opposite one another and each extending along an inclined plane for causing drops of treated wastewater received on the secondary surfaces to flow in a direction towards the main surface.
The biofiltering device according to the invention enables one to efficiently treat incoming wastewater irrespective of the flow rate thereof.